Titanium dioxide (titania) is extensively used in pigments, inorganic membranes, semi-conductors, and as a photocatalyst in gas and water purification processes. More than two million tons of titanium dioxide are produced annually; most of it is manufactured by aerosol processes which provide the advantages of small particle size, narrow size distribution, nearly spherical particles and high purity. Aerosol processes also are energy efficient and avoid the treatment of large liquid volumes associated with traditional wet chemistry processes. On the other hand, conventional aerosol processes are very complex and involve many physicochemical phenomena and mechanisms, such as chemical reaction, particle nucleation, condensation, coagulation, aggregation, heat and mass transfer, and thermophoresis. The fundamentals of these processes are not well understood and, as a result, the processes are difficult to control precisely. This makes design, operation and control of industrial reactors to carry out these processes more of an art than a science, relying heavily on experience and empiricism.
The use of photocatalysis as a purification process for both gas and liquid media has been of growing interest over the past decade. The photocatalytic destruction of organic contaminants is simple, low cost and fast and, therefore, attractive for a variety of purification applications. This process involves illumination of catalytic particles with near UV-light to promote photoexcitation of valence band electrons and holes. These electrons and holes migrate to the surface of the catalytic particles and participate in reduction/oxidation (redox) reactions with adsorbed species. These redox reactions produce highly reactive hydroxyl radicals which are responsible for the oxidation and in some cases the mineralization (complete oxidation to carbon dioxide, water and/or HCl) of the organic species. Among the oxide semiconductors which have been used for the photocatalytic destruction of organic pollutants, the anatase crystalline phase of titanium dioxide is the most effective because of its high photoactivity and stability.
Much of the current work on photocatalysis has been done using commercially available titanium dioxide powders. Though these powders are, in some cases, treated to improve their photocatalytic properties, they are generally not originally produced for this application. As a result, the most important properties for photocatalysis, i.e., specific surface area and crystalline phase composition, are not generally optimized during powder manufacture. Further, as discussed above, many of the processes currently used to produce titanium dioxide are only able to control such characteristics as surface area and phase composition with relative difficulty and little precision.
It was in this context that the present invention was made. This invention provides an effective, easily controlled process for preparing titanium dioxide (and other ceramic) powders. Particularly, the present invention allows for the effective controlling of the level of anatase phase and the surface area of the powder formed. By being able to vary and optimize anatase content and surface area independently of each other, with relative ease, the materials produced are useful for any use of titanium dioxide and are particularly useful as catalysts for photooxidation reactions.
Formenti, M., et al, in Aerosols and Atmospheric Chemistry, G. M. Hidy, ed., Academic Press, New York, pages 45-55 (1972), prepared titanium dioxide particles from the oxidation of TiCl.sub.4 in an oxygen-hydrogen diffusion flame. TiCl.sub.4 was introduced into the reaction by aspiration making flow rates difficult to control. They found that the morphology of the particles formed was a function of precursor concentration and residence time in the flame. Dopants and electric fields were not used in the preparation of titanium dioxide.
George, A. P., et al, Farad. Symp. Chem. Soc., 7: 63 (1973), investigated titanium dioxide production in premixed flames and found that the product particles had a self-preserving size distribution.
Great Britain Patent Specification 2,252,707, Tioxide Group Services, Ltd., published Aug. 12, 1992, describes a process for the decomposition of degradable organic materials (e.g., chlorophenol) using UV light and a photodecomposition catalyst which comprises a disk having anatase titanium dioxide adhered to it. It is disclosed that the titanium dioxide preferably has a high surface area in the range from 20-200 m.sup.2 /gm. No process is disclosed for making this titanium dioxide material. See also, U.S. Pat. No. 5,163,626, Urwin, et al, issued Nov. 17, 1992.
Ollis, et al, Environ. Sci. Technol., 25 (9): 1523 (1991), describes the use of photocatalysis to destroy contaminants in water. Titanium dioxide is taught to be an effective photocatalyst but there is little discussion of the physical characteristics of the titanium dioxide used. It is taught that the titanium dioxide used has a particle size of from 0.1 to 30 .mu.m.
Okamoto, et al, Bull. Chem. Soc. Jpn., 58: 2023 (1985), discusses the photocatalytic decomposition of phenol using anatase titanium dioxide powder. The average particle diameter of the powder is between 0.76 and 1.88 .mu.m.
U.S. Pat. No. 4,892,712, Robertson, issued Jan. 9, 1990, describes a reactor for fluid purification using photocatalysis. Anatase titanium dioxide is taught as being useful as a photocatalyst. However, there is no specific discussion of the physical characteristics of the titanium dioxide used, and specifically there is no discussion of surface area.
U.S. Pat. No. 5,198,403, Brand, et al, issued Mar. 30, 1993, discusses the production of a catalyst material using titanium dioxide which is completely or predominantly in the anatase phase and has a surface area of from 40 to 500 m.sup.2 /gm, preferably from 75 to 150 m.sup.2 /gm.
The introduction of ions into the reactants during production of carbon black has been taught to be effective in reducing the particle size of the product formed. The ions can be produced either by the addition of alkali metals to the reaction stream (Haynes, et al., Proceedings of the Seventeenth Symposium (International) on Combustion, The Combustion Institute, 1365 (1979)) or by using an electric/magnetic field (Soviet Patent 1,781,260, issued Dec. 15, 1992). Neither of these disclosures suggests that the presence of ions has any effect on the crystalline structure of the carbon black product.
It has also been suggested that by applying an electric field to ionize reactants before they enter the combustion area in the production of silicon dioxide, small particle size product may be produced. See, Soviet Patent 948,881, issued Aug. 7, 1982, and Hardesty and Weinberg, Proceedings of the Fourteenth Symposium (International) on Combustion, The Combustion Institute, 907 (1973). There is no teaching in these disclosures that this technique would have any effect on the crystalline structure of the silicon dioxide product.
The production of titanium dioxide in a diffusion flame reactor using an electric field applied by fixed flat screen electrodes located outside the combustion area has been taught. See, Katz, et al., Proceedings of the Twenty-Third Symposium (International) on Combustion, The Combustion Institute, 1733 (1990). This research suggested that the use of such electric fields resulted in increased particle size of titanium dioxide.